Need help with this coin

I went through my old half dollars and in my 1967s I noticed that the 6 in the date is missing. You can barely see that it should have been a 6 and it's missing. Is this an error coin or what do you think?

 

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Good morning Gabrielle and welcome to Coin Talk. Your Kennedy half dollar is a grease filled die. The good news is it’s 40% silver. Worth the silver value,no more. About $2

 

What does grease filled mean

 

The lubricants used with machinery at the mint sometimes causes some details to be obscured. Common occurrence.

 

http://www.error-ref.com/struck-through-smooth-viscous-material-grease-oil/